Lamellas make it easy
The Cumwhinton water treatment plant is benefiting from the use of lamellas manufactured by Nordic Water in Sweden that are simpler to operate and maintain.
Carlisle is a city of around 70,000 on the North-west coast, just south of the Scottish border. A shortfall in drinking water supply for the Carlisle area was identified in 1998. There followed a six-year period of consultation, environmental studies, planning and design.
The scheme involved a £16M upgrading of the existing water treatment plant at Cumwhinton both to increase capacity and to reduce the risk of crytospordium.
The flow for the plant is taken predominately by pumped extraction from the nearby Eden River, supplemented by gravity flow from the Castle Carrock Storage Reservoir. The plant has a designed maximum flow of 27MlD, a minimum of 12MlD and an average flow of 20MlD.
Matt Allason, process controller at the Cumwhinton plant, explains: “Currently, the proportion of our inflow is five parts from the river to one from the reservoir. The reservoir water has the advantage that there’s no cost to pump it. It tends to have higher colour from the peaty soils around the reservoir, so, from a quality point of view, I’d prefer river water only.”
At the raw water inlet the flow is monitored for; pH, colour, turbidity and conductivity. The first stage of treatment is dosing with an aluminium sulphate coagulant and a polyelectrolyte flocculant aid to the inflow, plus sulphuric acid to control its pH level.
Allason says: “pH is probably the most critical variable for efficient treatment – it affects the optimality of the flocc formation. The pH of our raw water here varies seasonally from 7 to 9. We dose the sulphuric acid to bring this down to a pH of 6.1 to 6.6; we aim for the higher figure in cooler weather.”
An inlet chamber splits the flow into three streams. The three-stream flow moves on to a flocculator stage and then into Johnson Lamella separators, mounted in concrete tanks. These lamellas were manufactured by Swedish company Nordic Water, and operate as follows: the main structure is a series of parallel plates, tilted backwards, at an angle of 55°, from the direction of flow.
The units have a patented flow control system, which ensures that the flow is distributed upward and uniformly across the plate assembly. This works by creating a pressure drop in collection channels above the plates, which ensures that the full area of each plate is covered by the flow. As the water flows upward, the floccs and other solids in the water settle on the plates and slide down into the bottom of the tank.
The sludge that accumulates at the bottom of the tanks is removed by Zickert bottom scrapers, also provided by Nordic Water. These work via the forward and return movement of wedge-shaped, concave sections: the concave side of the scraper bars acts like a bulldozer to push the sludge forward in the direction of a sludge pit, during a slow forward movement. The scraper bars are attached to flat steel, ‘drawing’ bars, which rest on polyethylene slides. The scraper movement is generated by an electric motor which in turn is connected to the drawing bars via a lever system. On the return movement the wedge-shaped section of the scraper slides under the sludge layer, without disturbing the sedimentation process.
From the lamellas, the clarified water moves onto rapid gravity, dual media sand filters and then to a contact tank where chlorine gas is dosed as a disinfection stage. Sodium Hydroxide is added to raise the final water pH to 7.5. After final measurements of levels of pH, chlorine and other monitoring to ensure compliance with all legal requirements, the main flow is supplied by gravity to Carlisle and surrounds, while a smaller flow is pumped to the community of Caldbeck.
The sludge removed from the raw water lamellas is directed to a second, sludge disposal treatment process at the plant. This is designed to dewater and thicken the sludge to the maximum extent. Water extracted from this process is fed back to the raw water inflow to the plant, while the thickened sludge accumulates in two holding tanks awaiting road transport from the plant to be further de-watered. One of the main stages in this second, sludge-washwater treatment process, consists of two additional Nordic Water Lamella separators, though in this case housed in conical steel containers, with sludge hopers at the bottom, and thus having no need for the bottom scrappers required in the rectangular concrete tanks of the raw water Lamellas.
Allason describes these units as superb. “The advantage of the lamella units is that they are so much easier to operate and maintain than the settlement equipment we had here at Cumwhinton before the upgrade in 2004.”
Prior to the upgrade, raw water settlement employed a method known as ‘Upward-Flow, Flat-Bottom, Flocc-Blanket Clarification’. Following flocculation, the inflow entered a rectangular tank from a series of inlets at the bottom. This upward flow lead to the formation of a flocc blanket suspended below the surface of the water.
The blanket moved gradually, over a separating wall, into a sludge pit at the far end of the tank. “The big problem with this system was restarting it after a drain-down: the sludge had to reform before the system was effective again and this could take anything from two days to two months.
With the lamellas, restarting is reliably achieved in a matter of hours. Also, the actual cleaning out process is easier and less labour-intensive, especially in the wash-water lamellas, where thanks to the sludge hoppers at the bottom, it’s quick and easy to hose down the unit and drain all the waste out.”